When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have abnormal cardiac electrical conduction patterns and irregular cardiac rhythms that are referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treatment includes use of a cardiac rhythm management system. Such systems are often implanted in a patient and deliver electrical stimulation therapy to the patient's heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacemakers deliver timed sequences of low energy electrical stimuli, called pacing pulses, to the heart, typically via one or more intravascular leadwires or catheters (referred to as “leads”) each having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pacing pulses (this is referred to as “capturing” the heart). Pacemakers also sense electrical activity of the heart in order to detect depolarization signals corresponding to the electrical excitation associated with heart contractions. This function is referred to as cardiac sensing. Cardiac sensing is used to time the delivery of pacing pulses with the heart's intrinsic (native) rhythm. By properly timing the delivery of pacing pulses, the heart can be induced to contract in a proper rhythm, greatly improving its output of blood. Pacemakers are often used to treat patients with bradyarrhythmias (also referred to as bradycardias), that is, hearts that beat too slowly. For that application, the pacemakers may operate in an “on-demand” mode, such that a pacing pulse is delivered to the heart only in absence of a normally timed intrinsic contraction. The on-demand pacing function is often embodied in algorithms exhibiting pace inhibition, in which pacing in a lead is prevented (inhibited) for one heart beat when a cardiac depolarization is detected in the same lead prior to the pace. In bradycardia patients, for example, on-demand pacing can ensure that pacing pulses are delivered only when the patient's intrinsic heart rate drops below a predetermined minimum pacing rate limit, referred to as a lower rate limit (LRL). Some pacemakers provide for two lower rate limits, a first LRL, sometimes called a normal LRL, to provide a minimum necessary heart rate during awake or exercise periods, and a second LRL, sometimes called a hysteresis LRL, to allow the heart to reach naturally slower rates during sleep. When the patent's heart rate falls below the hysteresis LRL, the pacemaker switches to the normal LRL to ensure the patient will have sufficient cardiac output by protecting the patient against abnormally slow heart rates.
Cardiac rhythm management systems also include cardioverters/defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias (also referred to as tachycardias), that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart is not allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia and allows the heart to reestablish a normal rhythm for efficient pumping of blood.
Cardiac rhythm management systems also include, among other things, pacemaker/defibrillators that combine the functions of pacemakers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is the treatment of congestive heart failure (also referred to as “CHF”). CHF, which can result from long-term hypertension, is a condition in which the muscle in the walls of at least one of the right and left sides of the heart deteriorates. By way of example, suppose the muscle in the walls of the left side of the heart deteriorates. As a result, the left atrium and left ventricle become enlarged, and the heart muscle displays less contractility, often associated with unsynchronized contraction patterns. This decreases cardiac output of blood, and in turn, may result in an increased heart rate and less resting time between heart contractions. This condition may be treated by conventional dual-chamber pacemakers and a new class of biventricular (or multisite) pacemakers that are termed cardiac resynchronization therapy (CRT) devices. A conventional dual-chamber pacemaker typically paces and senses one atrial chamber and one ventricular chamber. A pacing pulse is timed to be delivered to the ventricular chamber at the end of a programmed atrio-ventricular delay, referred to as AV delay, which is initiated by a pace delivered to or an intrinsic depolarization detected from the atrial chamber. This mode of pacing is sometimes referred to as an atrial tracking mode. The heart can be paced with a shortened AV delay to increase the resting time between heart contractions to increase the amount of blood that fills the ventricular chamber, thus increasing the cardiac output. Biventricular or other multisite CRT devices can pace and sense three or four chambers, usually including the right atrial chamber and both right and left ventricular chambers. By pacing both right and left ventricular chambers, the CRT device can restore a more synchronized contraction of the weakened heart muscle, thus increasing the heart's efficiency as a pump. When treating CHF either with conventional dual-chamber pacemakers or CRT devices, it is critical to pace the ventricular chambers continuously to shorten the AV delay or to provide resynchronizing pacing, otherwise the patient will not receive the intended therapeutic benefit. Thus the intention for treating CHF patients with continuous pacing therapy is different from the intention for treating bradycardia patients with on-demand pacing therapy, which is active only when the heart's intrinsic (native) rhythm is abnormally slow.
Conventional pacemakers and CRT devices in current use rely on conventional on-demand pacing modes to deliver ventricular pacing therapy. These devices need to be adapted to provide a continuous pacing therapy required for treatment of CHF patients. One particular problem in these devices is that they prevent pacing when the heart rate rises above a maximum pacing limit. One such maximum pacing limit is a maximum tracking rate (MTR) limit. “MTR” and “MTR interval,” where an “MTR interval” refers to a time interval between two pacing pulses delivered at the MTR, are used interchangeably, depending on convenience of description, throughout this document. The MTR presents a problem particularly for CHF patients, who typically have elevated heart rates to maintain adequate cardiac output. When a pacemaker or CRT device operates in an atrial tracking mode, it senses the heart's intrinsic rhythm that originates in the right atrial chamber, that is, the intrinsic atrial rate. As long as the intrinsic atrial rate is below the MTR, the device will pace one or both ventricular chambers after an AV delay. If the intrinsic atrial rate rises above the MTR, the device will limit the time interval between adjacent ventricular pacing pulses to an interval corresponding to the MTR, that is, ventricular pacing rate will be limited to the MTR. In this case, the heart's intrinsic contraction rate is faster than the maximum pacing rate allowed by the pacing device so that after a few beats, the heart will begin to excite the ventricles intrinsically at the faster rate, which causes the device to inhibit the ventricular pacing therapy due to the on-demand nature of its pacing algorithm. The MTR is programmable in most conventional devices so that the MTR can be set above the maximum intrinsic atrial rate associated with the patient's maximum exercise level, that is, above the physiological maximum atrial rate. However, many patients suffer from periods of pathologically fast atrial rhythms, called atrial tachyarrhythmia. Also some patients experience pacemaker-mediated tachycardia (PMT), which occurs when ventricular pacing triggers an abnormal retrograde impulse back into the atrial chamber that is sensed by the pacing device and triggers another ventricular pacing pulse, creating a continuous cycle of pacing-induced tachycardia. During these pathological and device-mediated abnormally elevated atrial rhythms, the MTR provides a protection against pacing the patient too fast, which can cause patient discomfort and adverse symptoms. Thus, to protect the patient against abnormally fast pacing, the MTR often is programmed to a low, safe rate that is actually below the physiological maximum heart rate. For many CHF patients with elevated heart rates, this means that they cannot receive the intended pacing therapy during high but physiologically normal heart rates, thus severely limiting the benefit of pacing therapy and the level of exercise they can attain. Therefore, there is a need for addressing this MTR-related problem in therapeutic devices for CHF patients as well as other patients for whom pacing should not be suspended during periods of fast but physiologically normal heart rates.